For a surge arrester placed between phase and ground, an arrester failure in all probability will result in a permanent ground fault. The arrester must then be disconnected or be replaced before the system can be energized again.
For arresters in stations, the relatively short shutdown required for a replacement is acceptable in view of the low risk of an arrester failure. In addition, the risk of operating the system without arrester protection for vital components in the station cannot be taken. Thus, a three-phase surge arrester in a station must be replaced by a new arrester.
The situation is somewhat different for surge arresters placed in the towers of a power line to protect against back flashover caused by lightning, or against switching surges. A failure of such an arrester resulting in a permanent ground fault may entail a lengthy shutdown before the defective arrester has been located and replaced. On the other hand, it is usually acceptable that the arrester is disconnected only. Therefore, if the arrester is provided with a cut-out device which automatically disconnects the arrester in the event of a failure, the operation of the line may continue and any defective arresters be located and replaced at some suitable time.
Surge arresters with cut-out devices are previously known, for example from the U.S. Pat. Nos. 2,305,436 and 4,308,566. However, these prior art cut-out devices have a complicated design, which renders manufacture difficult and increases the risk of malfunction.
The purpose of the present invention is to provide a surge arrester arrangement with a cut-out device which is simple and inexpensive to manufacture and which functions in a reliable manner. This is achieved by a cut-out device with the characteristic features described in claim 1. The cut-out device according to the invention includes a metal oxide block with a larger area than the blocks in the arrester, that is, with a higher relative energy capability. With such a design, the block in the cut-out device can only fail if the arrester connected in series with the device has failed, whereby incorrect tripping of the device is prevented.